Annular composite members and processes for producing the same

ABSTRACT

A composite, elongated annular cylindrical unit is comprised of a hollow metallic outer sheath, a hollow inner metallic sheath and a dense highly compacted solid disposed and enclosed therein. The composite member is formed by a rocking roll tube reducing process.

United States Patent inventors Appl. No.

Filed Patented Assignee Gerald R. Kilp Bethel Park;

Paul M. Bergstrom, Irwin; Harry M. Ferrari, Pittsburgh, all of Pa.

July 25, 1968 Jan. 1 l, 1972 Westinghouse Electric CorporationPittsburgh, Pa.

Original application Jan. 19, 1965, Ser. No. 438,450, now Patent No.3,409,973, which is a continuation-in-part of application Ser. No.120,066, June 27, 1961, now abandoned. Divided and this application July25, 1968, Ser. No. 747,667

ANNULAR COMPOSITE MEMBERS AND PROCESSES FOR PRODUCING THE SAME 9 Claims,3 Drawing Figs.

U.S. Cl 176/67, 176/83, 136/236 Int. Cl G2lc 3/02 Field of Search176/67, 82,

References Cited UNITED STATES PATENTS Went et a1.

Postal Blainey et a1. Storchheim Precht et a1 Butler et a1. Brauchler...Lederer...

Maxwell Takabashi et a1..

Takabashi et a1.. Danko et a1.

Allen et a1.

Precht et a1 Primary ExaminerCarl D. Quarforth Assistant ExaminerGarySolyst Attorneys F. Shapoe and C. L. Menzemer 29/1912 136/200X 176/83 X176/83 176/83 X 176/83 X 29/1912 X 136/202 X 176/83 X 176/83 X 136/202 X29/198 X 29/4743 X ABSTRACT: A composite, elongated annular cylindricalunit is comprised of a hollow metallic outer sheath, a hollow innermetallic sheath and a dense highly compacted solid disposed and enclosedtherein. The composite member is formed by a rocking roll tube reducingprocess.

ANNULAR COMPOSITE MEMBERS AND PROCESSES FOR PRODUCTNG THE SAME Thispatent application is a division of the copending application Ser. No.438,450 now U.S. Pat. No. 3,409,973 filed on Jan. 19, 1965 which, inturn, was a continuation-in-part of application Ser. No. 120,066, filedon June 27, 1961 now abandoned.

There is a need for elongated annular composite members comprising ahollow outer sheath or jacket of a metal and a hollow inner sheathjoined to an enclosed body of a highly compacted solid which latter maybe a ceramic, a semiconductor material, a nuclear fuel or the like.

For example, in producing thermoelectric devices, one of the mostdifficult problems is the application of good electrical contacts to abody of the thermoelectric material proper. The most efficientthermoelectric materials for both cooling and power generationapplications are almost always comprised of semiconductor or ceramiclikematerials. It is critically necessary that the electrical contacts whichare metallic, be joined or bonded to the thermoelectric material almostpreferably so that the lowest possible electrical drop occurstherebetween. Also, the contact member must be so mechanically orphysically joined that it will not loosen or become detached duringservice conditions when substantial temperature differences prevail inthe devices.

Those skilled in the art will appreciate the extreme difficulties insoldering, brazing or otherwise joining a metallic contact to asemiconductor or ceramic material, the latter often being brittle, toobtain these desired objectives. A high percentage of defective orunsatisfactory devices occur routinely even with the best processes nowin use.

During service many failures take place because of the gradual weakeningor mechanical disruption of the bond between the metallic contacts andthe body of thermoelectric material, whereby electrical continuity isdisrupted, and more importantly, the thermal conductivity will beseriously reduced.

The object of the present invention is to provide an annular compositeunit comprising a hollow inner cylindrical metal member and a hollowouter concentric cylindrical metal member disposed thereabout with acompactible material disposed in the space therebetween, the materialbeing highly consolidated to a dense mass and in firm and intimatecontact with the walls of the metal members.

Another object of the present invention is to provide an annularcomposite unit comprising a hollow inner cylindrical metal member and ahollow outer concentrical metal member disposed thereabout, both metalmembers having optimum compressive residual stresses and a preferredgrain orientation, with a compactible material disposed in the spacetherebetween, the material being highly consolidated into a dense massand in firm and intimate contact with the walls of the metal members.

Another object of the present invention is to provide a process forconcurrently consolidating a compactible nonmetallic thermoelectricmaterial into a thermoelectric body and to provide well-bonded metalcontacts thereto by simultaneously tube reducing a hollow outer metaljacket, a hollow inner metal jacket and the nonmetallic thermoelectricmaterial so as to produce an annular elongated thermoelectric element inwhich the metal jackets form electrical contacts extremely well bondedto the highly consolidated body of thermoelectric material capable ofmeeting requirements of optimum thermoelectric use.

A still further object of this invention is to provide a process forproducing annular nuclear fuel elements having both inner and outermetallic jackets of desirable optimum physical properties and preferredgrain orientation.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter.

For a better understanding of the nature and scope of the invention,reference should be had to the following detailed description anddrawings, in which:

FIG. 1 is an elevation view in cross section of an annular compositemember being processed in accordance with the teachings of theinvention;

FIG. 2 is an elevation view in cross section of an annular compositemember embodying the teachings of this invention; and

FlG. 3 is an elevation view in cross section of a composite member witha solid center embodying the teachings of this invention.

ln accordance with thepresent invention and in the attainment of theforegoing objects, there is provided a process for producing an annularcomposite, elongated cylindrical unit. The process comprises thedisposing of a compactible material between at least two concentriccylindrical metal members with the compactible material filling theannular space therebetween. This composite then is processed by arocking roll tube reducing process to effect a substantial reduction toprovide a dense, highly compacted, uniform body of material in finn andintimate contact with the walls of the metal members.

The preferred rocking roll tube reducing process employed by thisinvention comprises the use of the type of apparatus delineated on pg.276 in the text The Metallurgy of Zirconium by Lustman and Keize andknown as the Rockrite" process which has been utilized in the making ofseamless pipes. We have discovered unexpectedly that the rocking rollprocess is capable of producing the annular composite members havingrelatively unblemished surfaces on inner and outer metallic members aswell as a product which has an outstanding uniform quality and betterphysical characteristics than is now attainable by any known prior artmethod. This adaptation of the Rockrite rocking roll process also iscapable of cold compacting materials to a high density in annular formwhich previously could only be hot compacted to good density, such asthe iron sold under the trade name of RZ-Fe". This preferred rockingroll tube reducing process is also capable of compacting into a denseannular form materials, such as tungsten powder, which are difficult tocompact by any means, as an initial step prior to further working of thematerial.

This unique rocking roll tube reducing process for producing annularcomposite elements comprises applying a pair of cooperating rockingdies, having a roll working face configuration, about the periphery ofan elongated annular composite member comprising two metal tubes and acompactible material therebetween. inside the innermost hollow metallictube is positioned a stationary mandrel. The plane of the rocking motionof the dies is parallel to the longitudinal direction of the elongatedmember. The dies comprise working forces of progressively reducedcomplementary groove sections. During the member reducing operation, thedies both turn on their axes and they thereby progressively draw themember through the groove sections while exerting a powerful radiallydirected compressing action. The dies rock back and forth, the memberturning a small amount during each rocking cycle and the member advancesa predetermined distance into the rocking rolls, thereby reducing thecross-sectional area of a portion and concurrently increasing the lengththereof. The annular composite member is progressively worked by asimultaneous reduction of the thickness of the hollow inner and outermetallic members while compacting the material in the annular spacebetween the two metallic members with a high degree of uniformity anddensity.

This preferred rocking roll tube reducing process substantially differsfrom the process of swaging which has been employed in producing annularcomposite elements. The most common method of swaging employed is rotaryor hammer swaging. Rotary swaging reduces the cross-sectional area of amember by intermittent or continuous hammering of a hollow outsidemetallic member. Severe cold working results in this hollow outsidemetallic member necessitating stress relieving heat treatments after amoderate reduction to continually relieve the metal of the internalstresses which may split the member open. Before any substantial workingof the inside hollow metallic member can be accomplished, the materialin the annular space between the members must reach a mandrel density.The force accomplishing the reduction during hammer swaging is actuallya sharp blow momentarily applied to a small external surface of theoutside member which must be transmitted through the wall thickness ofthe member, then through the compactible material which is semifree toexert a flow pressure away from the point of maximum compression andthence to the hollow inner metallic member. This tends to push away thecompactible material and less densification occurs. The outer tubesurface bears the marks of these hammer blows and is otherwise rough andnonuniform in its properties.

Another method of forming an annular composite member is to swage theinternal portion of the hollow inside member while restricting anyradial movement on the part of the hollow outside member.

in both instances the only metallic member substantially cold worked isthat member to which the forces are directly applied. To limit theintermediate stress relieving steps required to achieve the maximumreduction in cross section areas before stress relief is required,however, swaging often is performed on members at an elevatedtemperature.

An important advantage obtained by employing the rocking roll tubereducing process of this invention, is that the hollow metallic memberscan be easily and controllably reduced in wall thickness to practicallyany design requirement, while simultaneously compacting at least onematerial in the annular space between the tubular metallic members to ahighly uniform material of a desired optimum density. By this process wecan readily secure an annular composite member up to 75 feet in lengthand 24 inches in outside diameter, with any desired annular materialdisposed therein.

This preferred rocking roll tube reducing process accomplishes largereductions in the cross-sectional area of annular composite memberswithout requiring any intervening stress relief annealing operations.Cross-sectional areas can be reduced by as much as 70 percent in onepass by this cold tube reducing process without stress relief anneal.Further reductions may be made employing two or more rocking roll tubereducing passes. Stress relieving of the metallic members between passesis not always required. In some cases heat treatment is only employed toachieve some optimum physical property for specific end product use.

During heat treatment to achieve an optimum physical property of thecladding material some sintering of the compactible material may occur.A partial sintering of the compactible material may be desirable and maybe included as a separate process step between two or more rocking rolltube reducing operations. This partial sintering will increase thedensity of the compactible material. The temperature for partialsintering must be so selected that the cladding material is notadversely affected. Temperatures at which the compactible material willsinter will be applied so that the outer tube metal is stress reliefannealed.

The employment of the rocking roll tube reducing process inmanufacturing annular composite elements yields other very importantbenefits. Since this process is essentially a squeezing operation, thecontact of the rolls on the metallic tubular member exerts forcespushing all the compactible material between the tubes from the leastdense to the most dense area. The result is that a higher uniformity ofdensity of the material is easily achieved during one tube reducingpass. Also both tubular metallic members are simultaneously anduniformly reduced whereby the walls of the metallic members are put incompression. Proper design of the tubes for the annular compositeelement has been found to lead to a preferred grain orientation in themetallic member. For example, zirconium tubes with a given grain texturewhen rocking roll treated will show a desired grain orientation notpossible with hammer swaging. Desirable crystalline orientation inthermoelectric materials can also be achieved while compacting the sameby the rocking roll process in the annular space between the metallicmembers.

The rocking roll tube reducing process is particularly suitable forcompacting metals, semiconductors, ceramics, thermoelectric and nuclearmaterials within two nested tubes.

A preferred product of this invention is a thermoelectric element madein accordance with the teachings herein. The thermoelectric material,such for example as lead telluride or germanium telluride, is introducedas a powder by tamping or vibration within the annular space between twoconcentrically disposed metallic cylindrical members. The annular spaceat each end of the tubes is plugged to retain the thermoelectricmaterial in place. The annular composite member is then rocking rollreduced to the final designed shape thereby cold working the metallicmembers to the reduced size while compacting the thermoelectric materialto dense optimum design requirements. The resulting elongated annularcomposite member may then be heat treated, if required, to ensure goodcontact between the compacted thermoelectric material and the metallicmembers.

The elongated annular composite member after being processed may then besevered into a plurality of cylindrical units which may be furthermachined to desired dimensions and shape. The elongated annularcomposite member may also be severed into a plurality of individualannular or other shapes of thermoelectric pellets with metal contacts inplace. These pellets, or cylindrical units, may then be joined to otherthermoelectric pellets by interposing insulating connections to producecomposite thermoelectric assemblies electrically connected in parallelor series. These composite thermoelectric elements and assemblies, whenproperly insulated both electrically and thermally, are then capable offunctioning as thermoelectric power generators or as cooling devices.

Another desirable use for this invention is in the manufacture ofannular nuclear fuel elements. The process for producing these reduced,high-density annular nuclear fuel elements is basically the same as thatindicated for the annular ther moelectric elements. The annular fuelelements made in this manner are superior in several ways to annularelements made by prior art techniques. The metallic members have athinner wall since both of the members are in compression. A thinnermetal wall is desirable in that less neutron absorption will occur. Theweight ratio of active nuclear fuel to metallic member, or cladding,becomes more favorable because the weight per unit foot of the claddingis reduced. At the same time the overall weight of a complete nuclearelement is reduced.

Another distinct advantage of the rocking roll process is theachievement of desired grain orientation in the cladding material, suchas in zirconium. Zirconium and zirconium alloy, such as zircaloy, cladnuclear elements can be produced with the majority of the basal latticeplanes parallel to the central axis of the element. This capabilityallows the walls of the cladding to be reduced in thickness, conservingweight and material, without sacrificing structural reliability sincethis crystal orientation is retained in spite of the severe reductionsemployed.

When properly designed, nuclear fuel elements produced by the rockingroll tube reducing process comprise a fissionable fuel material whichhas been easily compacted into a highly dense uniform structure inintimate contact with the cladding throughout the entire structure.These fuel elements are not as susceptible to the occurrence of claddingseparating away from the device compaction of fissionable material forthe creation of hot spots" within the element.

Suitable nuclear or fissionable fuel materials which may be compacted inthe annular space between concentric metallic members made fromstainless steel, zirconium alloys, aluminum and aluminum alloys, areboth arc-fused and ceramictype uranium dioxide, uranium nitride, uraniumcarbide and plutonium compounds and mixtures thereof, as well as metalpowders and any of these for example, mixtures consisting of percent byweight of stainless steel powder and 30 percent by weight of uraniumdioxide, or a mixture of 50 percent by weight of each of stainless steelpowder and uranium dioxide. Both ordinary and enriched fissionablematerial and fertile material, such as natural uranium, enricheduranium, plutonium and thorium as well as mixtures thereof, are capableof being compacted into annular fuel elements by the rocking rollreducing process.

It should be understood that the compactible material or materials maybe either cast or molded within the hollow cylindrical metal member.They may also be disposed therein in the form of one or more prepressedcompacts, or loosely as powder or flakes. In all cases, it is desirablethat as high a density of the material as is reasonably possible besecured when it is introduced into the annular space between the metalmembers. Suitable means known to those skilled in the art, such astamping and vibrating, may be employed to achieve reasonable densitiesfor the compactible material prior to any initial tube reductions.

For some applications, such as for thermoelectric members, the unitshould be evacuated to remove all gases from the material beingcompacted. For other applications such as melting electrodes, wherein analloyed component is placed in the annular space, evacuation is notrequired, or at most a nonreactive gas such as argon or nitrogen may bepassed through the assembly.

Referring to FIG. 1, there is shown schematically a portion of a rockingroll reducing apparatus having two rolls l2 and I4 having groundsurfaces, mounted on a reciprocating housing (not shown). In operation,an annular composite unit 16 comprising a hollow inner cylindrical metalmember 18 and an outer cylindrical metal member 20 with a compactiblematerial 22 disposed therebetween, is placed within complementarygrooves l3 and of progressively varying depth in rolls l2 and 14. Amandrel 24 is positioned in the hollow 26 of the member I8 through theback end thereof, the mandrel corresponding closely to the diameter ofthe hollow. In some cases the mandrel is tapered. The composite unit 16is then reduced by introducing it into the complementary grooves 13 and15 of the rolls l2 and 14 and by the rotary rocking motion of the rollsl2 and M in a plane parallel to the longitudinal direction of the unit16. The unit 16 is reciprocated within the grooves 13 and 15 in therolls l2 and i4 and advanced slightly at the end of each rocking cycleto progressively compact the unit 16 increment by increment. The unit 16also is rotated between each back and forth compacting movement so as toproduce uniform compaction. The apparatus 10 thereby progressivelyreduces the cross-sectional area of the unit 16 as it passesprogressively therethrough.

A reduction in cross-sectional area of up to 70 has been obtained in asingle stage by using the above-described apparatus and method.

Referring to FIG. 2, there is shown a section of a rocking roll tubereduced unit 30 consisting of an inner hollow cylindrical metal member32 and an outer hollow cylindrical metal member 34. A dense, uniform,highly compacted body 36 of material is disposed in the annular spacetherebetween, said body 36 being joined in firm and intimate contactwith the walls of the metal members 32 and 34.

in some cases a solid composite rod of materials can be processed.Referring to FIG. 3, there is shown a finally reduced unit 40 comprisingan inner cylindrical solid rod 42 and an outer hollow cylindrical metalmember 44 with a body of dense highly compacted material 46 disposedtherebetween and joined in firm and intimate contact with the walls ofthe metal rod 42 and metal member 44. In this modification, the solidrod 42 is not reduced as much as is member 44. Therefore only moderatereductions are secured.

The metals used in forming the inner and outer members of a givenannular unit are selected on the basis of their compatibility with thedesired compactible material and the sue to which the tube reducedannular unit will be placed. For example, when a compactiblethermoelectric material is employed, it is desirable to employ metalmembers selected from a group consisting of copper, aluminum. zirconiumor iron, or base alloys thereof, for the inner and outer metal members,or sheaths. These metals provide good electrical contact portions forthe thermoelectric material since they are relatively good electricallyand thermally conductive materials.

A mixture of molybdenum and zirconium powders, or other powder mixturesmay be compacted between copper, or other metallic cylinders. Thecylinders may then be removed and the compacted body employed as such.In a similar manner the formation of heavywalled tungsten tubing may bemade from tungsten powder by this process.

This rocking roll tube reducing process for manufacturing annularcomposite elongated members has other advantages over prior art methods.Ideally, ceramics, such as uranium dioxide, and intermetallic compounds,such as lead telluride and germanium telluride, should be readilycompacted to high densities in continuous rod or tube shapes many feetin length. At the same time it is advantageous to have apparatussuitable for rapid changing of dies from relatively large outside andinside diameter measurements to relatively small outside and insidediameter measurements. Both of these desirable features are availablewith this apparatus.

Other advantageous uses of the rocking roll reducing process can befound in the manufacturing of preinsulated members for heat exchangertubes, the producing of annular consumable electrodes, the compacting ofpowders into billets for further fabrication by extrusion, thefabrication of ceramic or high-temperature refractory metal tubes formuffler applications, the creation of carbide liners for cylinders andthe initial step in the fabrication of tubular or other products fromtungsten powders.

In producing preinsulated heat exchanger tubes, it is possible toeffectively insulate a metal tube by compacting a layer of ceramic orhightemperature refractory metal, such as mag nesium oxide, by therocking roll reducing process, between the tube proper and a thin metalcladding. When desirable, the cladding is removed after assuring theintegrity of the insulating coating by a technique such as firing orsintering.

Annular consumable electrodes are desirable in some manufacturingprocesses. Moreover, the cladding of the annular consumable electrodecan be specifically designed to become an alloying material in themelting process involved.

If the annular tube reduced unit is to be employed as a consumableelectrode and a compactible material comprising powdered or chipzirconium is to be employed, another metal such as molybdenum will beemployed for an inner and outer sheathing. The molybdenum may be desiredfor an alloying ingredient with zirconium.

intricately shaped tubes may be formed by the rocking roll tube reducingprocess. A suitable set of dies and a cooperating mandrel may bedesigned for the proper tube shape desired. The annular space betweenthe concentric cylindrical tube members is filled with a low meltingtemperature compactible material such as sodium chloride. The annularcomposite members are rocking roll tube reduced tothe desired shape andthe low melting material melted out upon subsequent heating.

The following examples are illustrative of the teachings of theinvention.

EXAMPLE] A stainless steel cylinder 78 inches long and 2.5 inches indiameter, of a wall thickness of 0.035 inch was sealed at one endthereof and fitted with a central stainless steel tube of l .73

inches outer diameter and a wall thickness of 0.035 inch. The innerannular space was filled with powdered sodium chloride. An and plug waswelded to the rear end of the stainless steel cylinder. The resultingunit was rocking roll reduced using the apparatus shown in FIG. 1 toeffect a 30percent reduction of area of the annulus between the tubesand simultaneously reduce the wall thickness of the tubes. The finaldimensions of this assembly were 1.6 inch outer diameter, 1.058 inchinner diameter, and 143 inches in length, and the wall thickness of eachtube was 0.029 inch.

The annular composite elongated member was then heated sufficiently tomelt the sodium chloride. The tubes were then removed from each otherand cleaned. They were clean and free of any surface blemishes or flaws.

EXAMPLE 11 An annular nuclear fuel element was prepared in the manner ofexample 1 except that uranium dioxide was substituted for the sodiumchloride.

The element was tube reduced to yield a 30 percent reduction in theannulus between the tubes. After this initial pass the uranium dioxidehad been compacted to a dense highly uniform structure of 85-88 percentof theoretical density.

Without any benefit of stress relieving, a second pass was made in thetube reducing apparatus. The annulus between the tubes was now 42percent of the original area. The uranium dioxide was now compacted to 9l .6 percent of its theoretical density. This is an excellent densityvalue. The surfaces or the tube were relatively unblemished and showedno structural flaws upon examination. Hammer or rotary swaging wouldhave created a rough surface and probable sheath defects withoutattaining the density for the uranium dioxide.

EXAMPLE III An annular elongated composite member was prepared in themanner of example I, the stainless steel tube having an outer diameterof 1.12 inches and the central tube having an inner diameter of 0.81inch. The inner annular space was filled with powdered magnesium oxide.The unit was rocking roll reduced so that the outer diameter of thestainless steel tube as reduced was 0.75 inch. The density of themagnesium oxide exceeded 70 percent, and it is known to be extremelydifficult to produce annular magnesium oxide bodies of density as highas this.

EXAMPLE IV A unit was prepared in the manner of example I, the outerdiameter of the outer stainless steel tube being 1.187 inches and aninner diameter of 1.222 inches. A stainless steel rod 1.187 01) wasfitted in the stainless steel cylinder and the inner annular space wasfilled with powdered iron sold under the trade name of RZ-Fe. The unitwas tube reduced so that the outer diameter as reduced was 0.75 inch.

The resulting density of this powdered iron was 88 percent oftheoretical density. This is an excellent density value.

EXAMPLE V An annular elongated composite member was prepared in themanner of example I. The outer diameter of the stainless steel tube was1.190 inches. The inner diameter of the stainless steel tube was 0.810inch. The annular space between the two stainless steel tubes was filledwith an iron powder sold under the trade name of RZFe.

Following a rocking roll tube reducing process of the annular compositeto an outside diameter of 0.750 inch, the density of the RZ-Fc ironpowder was found to be 93 percent of theoretical density. This is anexcellent density value, particularly since RZ-Fe iron powder isconsidered to be capable of hot compaction only.

EXAMPLE Vl An annular nuclear fuel element was prepared in the manner ofexample I, the stainless steel tube having an outer diameter of 1.19inches and the central tube having an inner diameter of 0.81 inch. Theinner annular space was filled with powdered uranium dioxide. Theelement was tube reduced so that the outer diameter of the stainlesssteel tube as reduced was 0.75 inch. The density of the uranium dioxideexceeded 88 percent.

EXAMPLE Vll An annular elongated composite member is prepared as inexample 11, using aluminum tubing, the space between the tubes beingfilled with N-type lead telluride, and, after tube reducing theresulting highly dense, compacted, elongated body is machined to cut offthe sealed ends.

The remainder of the tube is machined into short cylindrical pellets ineach of which the compacted lead telluride is joined to the inner andouter aluminum cylindrical sections.

A similar tube reduced annular elongated composite member is preparedsubstituting P-type germanium telluride for the lead telluride, andsimilar short cylindrical pellets are prepared. A series of alternategermanium telluridc and lead telluride pellets are joined into a longtubular structure whereby the inner tubes of every other pair ofadjacent pellets are electrically joined to each other, while the outertubes of each such joined pair are electrically connected to theadjacent pair, thereby producing a thermoelectric device. Passing a hotfluid such as water through the inner tubes and cooling the outer tubesof the device will cause an electrical potential to be produced betweenthe extreme ends of the device.

Similarly, other units may be prepared and rocking roll tube reduced asdescribed by substituting in the above example aluminum, zirconium,copper or iron for stainless steel, and other compactible materials maybe employed for the several compactible materials listed. Slightmodifications may be followed in the initial unit to compensate for thedifferent metal and design considerations. In particular, thermoelectricmaterials may be substituted for the various materials described in theexamples.

A plurality of concentric annular spaces may be present in each bynesting three or more tubes into a single unit, the several annularspaces being filled with different compactible materials. Thus threetubes of aluminum, for instance, of successively larger diameter aredisposed concentrically to provide two annular spaces and one space isfilled with zinc antimonide and the other space is filled with germaniumbismuth telluride. The resulting assembly is rocking roll reduced toprovide a composite body from which thermoelectric pellets may besevered in which zinc antimonide is in series with the germanium bismuthtelluride. More than three tubes may be employed in a similar manner.

The tubes may be coated with a layer of a material so as to improvebonding with the compacted material. Thus, iron plating on copper tubesis desirable to reduce any reaction from taking place as compared to thereaction of copper in contact with lead telluride during use of hethermoelectric devices so produced.

While the invention has been described with reference to particularembodiments and examples, it will be understood, of course, thatmodifications, substitutions and the like may be made therein withoutdeparting from its scope.

What we claim as our invention is:

1. An annular elongated cylindrical composite member comprised of a. atleast two spaced concentric cylindrical metal members of which theoutermost metal member has an essentially blemish-free surface, each ofsaid metal members having a high optimum residual compressive stress anda preferred grain orientation; and

a highly compacted, uniformly dense body approaching theoretical densityof compactible material disposed between each pair of spaced cylindricalmetal members, said body of compacted compactible material being inintimate physical contact with the walls of the metal members, thecomposite member being produced by a rocking roll process and not heattreated after manufacture to temperatures where the residual compressivestress is affected.

2. The annular composite member of claim 1 in which the innermostcylindrical metal member is a solid rod. 3. The annular composite memberof claim 1 in which the body of material consists metal particles.

4. The annular composite member of claim 1 in which the body of materialis a fissionable nuclear material.

5. The annular composite member of claim 3 in which each of the spacedcylindrical metal members is comprised of molybdenum, and

the body of material is at least one metal selected from the groupconsisting of powdered zirconium an zirconium chips.

6. The annular composite member of claim 1 in which each of the at leasttwo spaced concentric cylindrical metal members is comprised of amaterial selected from the group consisting of zirconium and zirconiumalloys.

7. The annular composite member of claim 4 in which each of the spacedcylindrical metal members is made of a metal selected from the groupconsisting of stainless steel,

zirconium, zirconium alloys, aluminum and aluminum alloys; and

the body of material is at least fissionable material one selected fromthe group consisting of arc-fused uranium dioxide. ceramic-type uraniumdioxide, uranium nitride, uranium carbide, plutonium, a plutoniumcompound, natural uranium, and enriched uranium.

8. The annular composite member of claim 6 in which the majority of thebasal lattice planes of each of the at least two .spaced concentriccylindrical metal members are parallel to the central axis of thecomposite member.

9. The annular composite member of claim 8 in which the materialcomprising the highly compacted, uniformly dense body of material is afissionable fuel material.

2. The annular composite member of claim 1 in which the innermost cylindrical metal member is a solid rod.
 3. The annular composite member of claim 1 in which the body of material consists metal particles.
 4. The annular composite member of claim 1 in which the body of material is a fissionable nuclear material.
 5. The annular composite member of claim 3 in which each of the spaced cylindrical metal members is comprised of molybdenum, and the body of material is at least one metal selected from the group consisting of powdered zirconium an zirconium chips.
 6. The annular composite member of claim 1 in which each of the at least two spaced concentric cylindrical metal members is comprised of a material selected from the group consisting of zirconium and zirconium alloys.
 7. The annular composite member of claim 4 in which each of the spaced cylindrical metal members is made of a metal selected from the group consisting of stainless steel, zirconium, zirconium alloys, aluminum and aluminum alloys; and the body of material is at least fissionable material one selected from the group consisting of arc-fused uranium dioxide, ceramic-type uranium dioxide, uranium nitride, uranium carbide, plutonium, a plutonium compound, natural uranium, and enriched uranium.
 8. The annular composite member of claim 6 in which the majority of the basal lattice planes of each of the at least two spaced concentric cylindrical metal members are parallel to the central axis of the composite member.
 9. The annular composite member of claim 8 in which the material comprising the highly compacted, uniformly dense body of material is a fissionable fuel material. 